<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        div{
            margin-top: 20px;
        }
        .hour{
            font-size: 25px;
            background:black;
            color:#fff;
            width: 200px;
            height: 70px;
            padding: 20px;
            margin-right: 10px;
        }
        .minute{
            font-size: 25px;
            background:black;
            color:#fff;
            width: 200px;
            height: 70px;
            padding: 20px;
            margin-right: 10px;
            
        }
        .second{
            font-size: 25px;
            background:black;
            color:#fff;
            width: 200px;
            height: 70px;
            padding: 20px;
        }
    </style>
</head>
<body><div>
    <span class="hour">10</span>
    <span class="minute">2</span>
    <span class="second">3</span>
    </div>
    <script>
        var hour=document.querySelector('.hour');
        var minute=document.querySelector('.minute');
        var second=document.querySelector('.second');
        var inputTime=+new Date('2020-6-10 24:00:00');
        countDown();
        setInterval(countDown,1000);
        function countDown(){
            var nowTime=+new Date();
            var times=(inputTime-nowTime)/1000;
            var h=parseInt(times/ 60 / 60 );
            h=h<10? '0'+h:h;
            hour.innerHTML=h;
            var m=parseInt(times /60 % 60);
            m=m<10? '0'+m:m;
            minute.innerHTML=m;
            var s=parseInt(times % 60);
            s=s<10? '0'+s:s;
            second.innerHTML=s;
        }
    </script>
</body>
</html>